Narrowband gain control of receiver with digital post filtering

ABSTRACT

An Automatic Gain Control (AGC) circuit as used in a digital receiver that utilizes a main loop filter that is of a relatively wide bandwidth. A pre-filter, wideband variance is determined from the input digital signal, and a post-filter, narrowband variance is also determined. The wideband and narrowband variances are then compared to determine if the wideband signal power indicates a variance level that is too great to permit normal loop operation. By reapplying this difference in the power levels to the filter output as needed, such as by a scaling operation, the loss in dynamic range is effectively recovered. In a preferred embodiment, an adjustable gain input amplifier feeds an intermediate frequency (IF) signal to an analog-to-digital converter (ADC). The digitized IF signal is then down-converted to a baseband frequency and subjected to digital filtering. A narrowband sample variance (P N ) of the digitally filtered (narrowband) data is then determined. A wideband sample variance (P W ) is also taken from the raw ADC output data over the same period as the time period used for P N . In the presence of out-of-band signal components, P W  will be quite different from P N . This difference indicates a desired proportional difference in a control voltage or a gain backoff amount.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/847,056, filed May 1, 2001. The entire teachings of the aboveapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to radio receivers in whichintermediate frequency signals are processed digitally, and morespecifically to an automatic gain control circuit that compensates foradded wideband signal power.

Modern radio receivers, such as used in cellular telephone, wirelessLocal Area Network (LAN), wireless Internet access systems, and similarequipment typically now employ digital signal processing techniques tosome degree. Digital signal processing permits the replacement ofphysically large, costly, and unpredictable analog filtering componentswith their digital counterparts. These receiver architectures requirehigh speed, wideband, analog-to-digital converts (ADCs) and digitalfilters. Present day ADC technology permits sampling at IntermediateFrequency (IF) or even greater frequencies. However, by replacingtraditional analog filters with digital filters implemented afterdigitization, the ADC potentially also samples out-of-band unwantedsignal components. These unwanted signal components may consist ofadjacent channels, extra noise, or even jamming signals along with thedesired signal of interest.

Direct application of analog receiver architectures to a digitalimplementation therefore, is often not sufficient to provide therequired signal filtering properties. One difficulty stems from the factthat analog demodulation techniques are not directly adaptable todigital receivers. For example, clipping of a received signal lowers theprobability of correctly detecting the signal of interest and the dataderived there from.

To reduce clipping, digital receivers often include one or more variablegain amplifiers that permit the gain of the receiver to be adjusted by afeedback control signal. The process of adjusting the received signal inthis fashion is called Automatic Gain Control (AGC). In the typicaldigital receiver, the AGC circuitry measures an output signal power ofthe variable gain amplifier. This measured value is then compared with avalue representing the desired signal power to derive an error signal.The error signal is then used to control the variable amplifier gain sothat the input signal strength coincides with the desired input signalpower. In the typical desired arrangement, the AGC circuit therefore mayhold the amplitude of the variable gain amplitude close to the fulldynamic range of the analog-to-digital converter.

In the presence of out-of-band signal components, standard gain controlloop architectures are often insufficient to guarantee properanalog-to-digital converter operation. Especially in the cellularenvironment, a digital receiver may receive signals that exhibit rapidand wide variation in signal power. For example, in Code DivisionMultiple Access (CDMA) wireless communications, it is necessary toprecisely control the power level of transmitted signals for propercapacity management.

Some have proposed the use of an AGC circuit wherein the filterbandwidth may be changed. In particular, as described in U.S. Pat. No.6,178,211, the filter coefficients of a digital signal processor areswitchable between a first wide bandwidth to a second narrowerbandwidth. A post-filter level detector is responsive to the filteredsignal and provides a control signal for selecting one of the banks offilter coefficients. Thus, the circuit reduces the effect of adjacentchannel interference by narrowing the bandwidth of a filter in thereceiver, which reduces the signal content from the adjacent channelpropagating through the receiver.

This type of circuit provides an effective method of filtering out ofband signals after the ADC. However, gain control in this circuit isbased entirely on the signal power present at the input to the ADC,rather, its digital output. This requires an adaptive filteringtechnique to switch to the proper coefficients as needed. Often, casesarise in which multiple sets of filter coefficients are not availabledue to signal processing or memory resource restrictions.

SUMMARY OF THE INVENTION

The circuit proposed herein benefits from reduced complexity by using afixed set of filter coefficients and switching control of the AGC loopfrom narrowband to wideband power calculations (and the reverse) asnecessary. Furthermore, backend gain control is achieved with a simplescale multiplier applied to both the in-phase (I) and quadrature (Q)data paths.

More particularly, the present invention is an architecture for anAutomatic Gain Control (AGC) circuit as used in a digital receiver thatutilizes a main loop filter that is of a relatively wide bandwidth. Apre-filter, wideband variance is determined from the input digitalsignal. In addition, a post-filter, narrowband variance is alsodetermined. The wideband and narrowband variances are then compared todetermine if the wideband signal power indicates a variance level thatis too great to permit normal loop operation. In such a case, thedynamic range of the desired signal components (e.g., the desirednarrowband signal) would otherwise be reduced. By reapplying thisdifference in the power levels to the filter output, such as by ascaling operation, the loss in dynamic range is effectively recovered.

In a preferred embodiment, an Automatic Gain Control (AGC) circuitincludes an additional wideband and narrowband variance comparisonsection. An adjustable gain input amplifier feeds an intermediatefrequency (IF) signal to an analog-to-digital converter (ADC). Thedigitized IF signal is then down-converted to a baseband frequency andsubjected to digital filtering. A narrowband sample variance (PN) of thedigitally filtered (narrowband) data is then determined. A widebandsample variance (PW) is also taken from the raw ADC output data over thesame period as the time period used for PN.

Assuming that the input signal fed to the ADC is relatively bandlimited, under normal operating conditions without much interferencesignal level, the digitally post-filtered signal level has relativelythe same power as the raw ADC output. In other words, the normalcondition is such that the digital filter removes only perhaps low-levelnoise and aliasing components generated from the down-conversionprocess.

In the presence of out-of-band signal components, the wideband samplevariance (PW) will be quite different from the narrowband samplevariance (PN). This difference indicates a desired proportionaldifference in a control voltage or a gain backoff amount. Once thisbackoff amount exceeds a predetermined level, that value is used in acontrol voltage calculation to reduce input signal.

However, simply replacing the narrowband variance with the widebandvariance in the control voltage calculation may yield an inaccuratereceived signal strength indication and, in turn, actually reduce thesignal level of the filtered data. Thus, this power level backoffvoltage is also converted to a scale value used to multiply or amplifythe filtered data.

This results in recovering the reduced signal level of the filtereddata. In addition, if the sampling or decimation rate is high enough,lost sample resolution can also be recovered by output filtering.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a high-level block diagram of an automatic gain controlcircuit that makes use of a wideband and narrowband variancedetermination according to the invention.

FIG. 2 illustrates a situation where narrowband interference reduces theavailable dynamic range at the input to the analog-to-digital converter.

FIG. 3 illustrates the affect of the present invention on the availabledynamic range.

FIG. 4 is a signal flow diagram representation of the automatic gaincontrol circuit.

FIGS. 5A, 5B, and 5C are a detailed circuit diagram of the automaticgain control circuit.

FIG. 6 is a more detailed view of the two-stage digital filter block.

FIG. 7 is a more detailed view of the five-stage digital filter block.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

Turning attention now to FIG. 1, there is shown a high-level blockdiagram of an Automatic Gain Control (AGC) circuit 10 operated inaccordance with the invention. The AGC circuit 10 includes a variablegain input intermediate frequency (IF) amplifier 12, ananalog-to-digital converter (ADC) 14, digital down-converter and filterblock 15 that includes quadrature mixers 16-i and 16 q as well as a pairof low pass filters 17-i and 17-q, associated with an in-phasequadrature signal path, a main AGC loop filter 18, and digital-to-analogconverter (DAC) 19. The variable gain amplifier 12, ADC 14, digitaldown-converter and filter 15, main ACG loop filter 18, and DAC 19 arestandard components of a digitally implemented Automatic Gain Controlcircuit.

A unique aspect of the present invention is to also include a widebandvariance block 22 coupled to the output of the ADC 14, a narrowbandvariance block 21 coupled to the output of the digital down-converterand filter 15, a comparison function 23 to compare the output ofwideband variance 22 and narrowband variance 21, as well as outputscaler 24. In general, the components that comprise the invention add avariance comparison section that determines the difference between anarrowband sample variance as represented by the signal samples at theoutput of the filters 17, and comparing that to a wideband samplevariance associated with the input to the digital down-converter andfilter 15.

The difference between the wideband sample variance (PW) as provided bythe wideband variance circuit 22 and narrowband variance value (PN) asprovided by the narrowband variance circuit 21 represents an indicationof whether the setting of the gain control loop is sufficient toguarantee proper operation of the ADC 14. In particular, when thedifference between the wideband sample variance PW and the narrowbandsample variance PN exceeds a predetermined level, an input power levelvoltage is provided to the main AGC loop 18 to cause it to limitclipping by the ADC 14.

Thus, in cases when the narrowband sample variance PN of the narrowbanddata is sufficiently the same as the determined wideband variance PW,then the loop operates as in the prior art as a standard automatic gaincontrol loop. However, when the power level of out-of-band signalcomponents is sufficiently large (as provided by jamming signals, noise,interfering adjacent channels, etc.), the difference in variance isdetected at the output of the comparison 23. In this instance, anadditional input power level voltage is provided to offset the AGC loopsetpoint input.

A scale value is also preferably provided at the output of thecomparison 23. The scale value allows the multiplier 24 to offset areduction in the power level of the filtered data during conditions whenthe wideband variance exceeds the narrowband variance. In systems withrelatively high decimation rates, even the lost sample resolution can berecovered, provided adequate filtering is applied after themultiplication.

Turning attention now to FIGS. 2 and 3, the situation addressed by theinvention is described in relative terms. In particular, in connectionwith FIG. 2, there is shown a signal power diagram for signals presentat the input of the ADC 14. In this instance, not only is a desiredsignal 30 present, but also there is present an out-of-band interferingsignal 31. The ideal power level setpoint for the analog-to-digitalconverter should be at a level 32. However, the presence of therelatively strong interference 31, that is, a signal that is muchstronger than the desired signal 30, causes a standard AGC loop toadjust itself to avoid clipping of the ADC 14 output. Thus, the desiredsignal 30 is suppressed in amplitude, causing reduced accuracy in RSSIcalculation and the like.

FIG. 3 shows a program in a situation as provided by the invention. Inparticular, using a comparison between the narrowband variance andwideband variance, the loop gain control signals are adjustedaccordingly such that the power of the desired signal is increased atthe output of the ADC 14. This results in a more accurate magnitude ofan output signal and results in overall improved receiver performance.

FIG. 4 is a signal flow representation of the AGC circuit 10. In thepreferred embodiment, an input signal scaled from a range of −57 throughzero decibels with respect to a millivolt (dBm) is provided to an inputadder 40. The adder 40 represents the operation of the variable gainamplifier that is controlled by the gain control input signal 70.

The amplifier output is fed to a lowpass filter 42. The differencecircuit 44 provides an estimate of the difference between the input andoutput voltages applied to the lowpass filter 42. A comparator 46 thencompares this difference to a predetermined threshold. In this instance,the predetermined threshold is set to −3 dBm or a half-power level. Ifthe output signal power provided by the lowpass filter 42 is less thanthe input signal power, then the jamming signal (JAM) is asserted. Inthis instance, it is concluded that jamming or interfering signal poweris present such that the power level that is between the in-band andout-of-band signals needs to be added back into the down-converter data.In this instance, the unfiltered input signal is selected by themultiplexer 48.

In an instance where the narrowband signal power is approximately thesame or greater than the broadband signal power, then the filteredsignal is used for the low control signal. An AGC setpoint is thencompared by comparator 50 and the signal is scaled by the gain constantK1. A loop filter 53 represented by the summer and delay 56 filters thisfeedback signal. Additional delay may be provided by a delay block 60.

The digital analog converter (DAC) is represented by a model 52 thatincludes log-to-linear converter, lowpass filter 66, and linear-to-logconverter 68.

The resulting signal is then fed as the gain control signal 70 back tothe input. Decimation, although not shown in FIG. 4, can be provided sothat any values larger than a predetermined value merely serve to scalethe data while maintaining the same relative resolution.

FIGS. 5A, 5B, and 5C are a detailed circuit diagram for one preferredembodiment of the AGC circuit 10. In this implementation, the variablegain amplifier 12 is seen to comprise three individual variable gainstages 120-1, 120-2, and 120-3.

The analog-to-digital converter (ADC) 14 is a 12-bit converter operatingat 58.9824 mega-samples per second (Msps), provided with a 2-voltpeak-to-peak input signal on a 70 MHz IF frequency. The sample rate of58.9824 Msps represents a rate of 48 times the known bandwidth of aninput signal that is 1.2288 MHz, which is typical for an input CDMAradio frequency signal.

The digital down conversion and filtering circuit 15 is implemented inan integrated circuit known as an AD6620 available from Analog Devicesof Wilmington, Mass. The device provides a 16-bit signal path for bothin-phase (I) and quadrature (Q) signal processing. The sampling rate isset at 4.9152 mega samples per second at baseband and an AGC update rateof 38.4 kilohertz. The AD6620 includes the input quadrature mixer 16-i,16-q, two cascaded integrator comb (CIC) type filters, and a RamCoefficient Filter (RCF).

FIG. 6 is a more detailed diagram of a two-stage CIC filter shown aselements 160-i-1 and 160-q-1. This filter provides decimation by afactor of three. For the preferred implementation, the filter has anequivalent Finite Impulse Response (FIR) as follows:

H2=[1, 2, 3, 2, 1]/16

The five-stage CIC5 filter designated as 162-i-1 and 162-q-1 has asimilar FIR-type response. This filter, however has five stages with aresponse as follows:

H5=[1, 5, 15, 35, 65, 101, 135, 155, 155, 135, 101, 65, 35, 15, 5,1]/1024

The cascaded CIC filters 160 and 162 provide a total factor-of-12decimation, resulting in a four times rate output (4.9152 Msps).

The RCF filter 164-i and 164-q is a 12-tap FIR filter with coefficients:

H12=[12512, −10490, −34369, −1668, 99965, 195803, 195803, 99965, −1668,−34369, −10490, 12512]/524288

An estimate of narrowband variance provided by the power leveldetermination circuit is implemented with the squarers 170-i, 170-q, andsummer 172, as well as accumulator 174. The accumulator 174 provides anaverage output power indication for every 128 samples, with theshift-out operation being controlled by a 1/32 times clock. The averagepower value output on signal line 175 is then fed to the differencingcircuit 180. The narrowband variance value is determined from componentsof the received RF signal across a bandwidth which is less than twice abandwidth of the intended received signal.

A wideband variance estimate is provided by taking the signal 140 outputfrom the ADC and feeding it to a squaring circuit 190. As in the casewith squaring circuits 170-i and 170-q, the squaring circuit 190 resultsin an output signal at twice the frequency of the input RF signal. Thewide band variance value is determined from components of the receivedRF signal across a bandwidth which is at least twice as wide as abandwidth of the intended receiver signal. The output of squaringcircuit 190 is then fed to an accumulator 192 that provides a sampleoutput every 1/32 clock period time. The accumulated power value is thenfed to a divider 194 and rounding 196 to provide an average value forthe wideband variance estimate. This value is then fed to the flip-flop198 to align it in time with the narrowband value.

Returning attention to generation of the narrowband variance value, thedifferencing circuit feeds a rounding circuit 182 and log table 184prior to being fed as the Received Signal Strength Indication (RSSI)value to the input of an A minus B comparator 200. The A minus Bcomparator 200 accepts the output of the log table 199 containing theaverage wideband power value at input B.

The differencing circuit 200 thus provides the narrowband minus widebandestimate that is needed to determine the value of the scale factor andother control factors on the loop. For example, the comparator 202compares the output of differencing circuit 200 to a (arbitrary) value0xFA00 that is a 16-bit hexadecimal representation of a −3 dB referencevalue multiplied by 512. This comparator 202 thus indicates whether thewideband value is greater than the threshold value, and if so, assertingthe JAM signal 203 to control the multiplexer 204 output to create thecrest signal 205.

Thus, if the narrowband variance signal is greater in magnitude than thewideband variance signal, then the narrowband signal is used to controlthe remainder of the loop. Otherwise, the signal is zeroed out and notpermitted to control the crest of the loop operation.

The crest signal 205 is then fed about to the AGC setpoint summer 208that is, in turn, fed to the loop filter. The loop filter in thisembodiment consists of the multiplier 210, gain input 212, summer 214and delay 216. The loop output signal 217 is then ingested by anattenuation factor to provide attitude 220 to provide an attenuatedoutput signal. This is then fed to a gain distribution circuit 212 thatprovides various signals to control the operation of the D to Aconverters (DACs) 214. The DACs 214 provide signals IF_AGC1 and IF_AGC2to control the respective variable gain amplifiers 120-1, 120-2, and120-3.

Since the gain control voltage in the loop filter is linear-linear,filtering the RMS power level in decibels is possible. Otherwise, ifalternate formats are used, the data needs to be converted to voltsprior to the integration process performed by the local filtercomponents 214 and 216.

A crest value 205 is also fed to an inverse log table 240 to provide thescale factor 241. The scale factor is then used as an input to themultiplier 173 that provides the visual received data output on both Iand Q channels.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for automatic gain control in a circuit that outputs adigitized signal, comprising: coupling an input signal to a variablegain amplifier, the variable gain amplifier producing a gain controlledsignal in accordance with a gain control input; digitizing the gaincontrolled signal to produce a digital signal; determining a widebandvariance value from the digital signal; determining a narrowbandvariance value from the digital signal; if the narrowband variance valueis less than the wideband variance value, using the narrowband variancevalue to set the gain control input on the variable gain amplifier; andif the wideband variance value is greater than the narrowband variancevalue, comparing the narrowband and wideband variance values to furtherdetermine operating parameters of the variable gain amplifier.
 2. Themethod of claim 1, additionally comprising the step of: filtering thedigital signal to produce a filtered signal; determining the narrowbandvariance value from the filtered signal.
 3. The method of claim 2,wherein the signal is a baseband signal.
 4. The method of claim 2,additionally comprising the step of: down-converting the digital signal,to produce a down-converted signal; quadrature demodulating thedown-converted signal, to produce an in-phase (I) and quadrature (Q)signal used in determining the narrowband variance value.
 5. The methodof claim 1, wherein the wideband variance value is determined directlyfrom the input signal.
 6. An automatic gain control apparatus for use ina radio frequency receiver that outputs a down-converted, digitizedsignal, the apparatus comprising: a variable gain amplifier coupled toreceive a radio frequency (RF) signal, the variable gain receiveramplifier having a gain control input, and to produce a gain controlledRF signal; a digitizer, connected to digitize the gain controlled RFsignal to produce a received digital signal; a wideband variancedetector, for determining a wideband variance value from the receiveddigital signal; a narrowband variance detector, for determining anarrowband variance value from the received digital signal; acomparator, for comparing the wideband variance value and narrowbandvariance value, to set a reference level for the automatic gain controlloop circuit, wherein the narrowband variance value is connected to setthe gain control input on the gain controlled receiver; and acomparator, for comparing the narrowband and wideband variance values todetermine a scale factor for the input to the variable gain amplifier.7. An apparatus as in claim 6 wherein the wideband variance value isdetermined directly from the received digital signal.
 8. An apparatus asin claim 6 additionally comprising: a down-converter, fordown-converting the received digital signal, to produce a down-convertedsignal; a filter, connected to receive the down-converted signal, and toproduce a filtered received signal; and wherein the narrowband variancedetector determines the narrowband variance value from the filteredreceived signal.
 9. An apparatus as in claim 6 wherein thedown-converted signal is a baseband signal.
 10. An apparatus as in claim6 additionally comprising: a quadrature demodulator, connected toreceive the down-converted signal, and to produce an in-phase (I) andquadrature (Q) signal used in determining the narrowband variance value.